1. Field of the Invention
The present invention relates generally to a wireless communication system and, more particularly, to an orthogonal frequency division multiplexing channel estimation method and system in a time-varying environment in which a transmitter and/or receiver moves at high speed.
2. Description of the Related Art
Finite communication bandwidth is a significant limitation of the capacity of wireless communication systems. Accordingly, in order to increase the wireless communication capacity of the wireless communication systems, orthogonal transmission methods, such as Orthogonal Frequency Division Multiplexing (OFDM), have been developed to modulate information onto orthogonal subcarriers and transmit the modulated signal.
OFDM is a broadband modulation method of dividing a frequency bandwidth allocated for a communication session into a plurality of narrow frequency subbands, in which each of the subbands includes a Radio Frequency (RF) sub-carrier and each of the subcarriers is mathematically orthogonal to RF subcarriers included in the other sub-channels. The orthogonality of the subcarriers allows their spectra to overlap each other without interference with the other subcarriers. Accordingly, the OFDM has a high data transmission rate and very efficient use of bandwidth is possible because the bandwidth is divided into a plurality of orthogonal subbands.
FIG. 1 is a block diagram illustrating an example of a conventional OFDM communication system 100. The OFDM communication system 100 includes a transmission side 102 through 118 and a reception side 112 through 136.
On the transmission side, a data transmission unit 102 inputs data, which is generally a bitstream, to an encoder 104. The encoder 104 applies error correction code (generally, forward error correction code) to the bitstream, and transfers the encoded bitstream to a symbol mapper 106. The symbol mapper 106 divides the bitstream into groups of P bits (P-tuples) and then maps each P-tuple to one symbol which is chosen from M constellation points to generate a next symbol stream. In this case, M=2P, and each symbol is represented as one point selected form a group of points in multi-dimensional modulation. Generally, two-dimensional modulation, such as Multiple Phase Shift Keying (MPSK) or Multiple Quadrature Amplitude Modulation (MQAM), is used as a symbol mapping scheme.
The symbol mapper 106 transfers the symbol stream to a Serial-Parallel (S/P) converter 108, such as an inverse multiplexer. The S/P converter 108 converts the symbol stream from a serial form to a parallel form, and applies the output of N parallel symbols to an orthogonal modulator 110, such as an Inverse Fast Fourier Transform (IFFT) block that is a fast form of an Inverse Discrete Fourier Transform (IDFT). In order to generate N parallel-modulated subcarriers, the orthogonal modulator 110 modulates each of the N subcarriers using one of the N symbols. In this case, each subcarrier is orthogonal to the other subcarriers. Thereafter, the N modulated subcarriers are transferred from the orthogonal modulator 110 to a Parallel-Serial (P/S) converter 112, such as a multiplexer for combining the N modulated subcarriers together, to generate an output signal 113. The P/S converter 112 transmits the output signal 113 to a Cyclic Prefix (CP) adder 114 that adds a guard band interval or cyclic prefix to the signal 113 to generate an output signal 115. Thereafter, the output signal 115 is transferred to a frequency up-converter 116 that converts the output signal 115 from a baseband frequency to a transmission frequency. The frequency up-converted signal is transferred to a Power Amplifier (PA) 118 that amplifies the signal and transmits the amplified signal through an antenna.
The reception side 120 through 136 implements a function inverse to that of the transmission side 102 through 118. A received signal is transmitted to a Low Noise Amplifier (LNA) 120 that amplifies the received signal, and then the amplified signal is transferred to a frequency down-converter 122 that converts the amplified signal from a transmission frequency to a baseband frequency. Thereby, the frequency down-converter 122 outputs a baseband signal. The baseband signal is transferred to a CP eliminator 124 that eliminates the cyclic prefix added to the baseband signal. The CP eliminator 124 transfers a cyclic prefix-free signal to an S/P converter 126. The S/P converter 126 converts the frequency down-converted prefix-free signal from a serial form to a parallel form, and outputs N parallel-modulated subcarriers. The N parallel-modulated subcarriers are transferred to an orthogonal demodulator 128, such as a Discrete Fourier Transform (DFT) or Fast Fourier Transform (FFT) block, that demodulates transmitted information based on the N orthogonal functions that are used in the orthogonal modulator 110. The output of the orthogonal demodulator 128 includes N parallel symbols corresponding to the N modulated subcarriers, which are transmitted to a P/S converter 130. The P/S converter 130 converts the symbols from a parallel form to a serial form to generate a symbol stream, and transfers the generated symbol stream to an inverse symbol mapper 132. The inverse symbol mapper 132 generates a bitstream by restoring the P-tuples corresponding to each of the symbols, based on the symbol mapping rule or scheme used by the symbol mapper 108. Thereafter, the inverse symbol mapper 132 transmits the restored bitstream to a decoder 134. The decoder 134 decodes the bitstream based on the error correction code that has been applied by the encoder 104, and transfers the decoded bitstream to a data reception unit 136.
The key to the bandwidth efficiency of the OFDM system is the orthogonality of the subcarriers. In order to maintain the orthogonality of the subcarriers, the OFDM system adds the guard band interval having a time length Tg, which is designated as a cyclic prefix, to each of the OFDM symbols. Accordingly, since the transmitted OFDM symbol can be generally regarded as including two intervals, that is, the guard band interval Tg and an OMDM symbol interval Ts, the total period of the transmitted symbol is Ttotal=Tg+Ts. In the other hand, the use of the guard band interval or the cyclic prefix reduces spectrum efficiency because additional time is spent due to the repeated parts of information. Therefore, the length of the guard band interval should be limited.
However, in order to eliminate interference between symbols (a symbol transmitted through a subband interferes with the following symbol transmitted through the same subband), the guard band interval should be lengthened by at least a period corresponding to multipath delay or fading caused in a system by a propagation environment.
In a wireless communication system, it is difficult to predict the multipath delay. The mutipath delay in such a system is a random variable, and there are some cases where the mutipath delay is longer than the predetermined length of the cyclic prefix, in the wireless communication system.
That is, the OFDM system for packet-based transmission uses relatively short OFDM symbols to perform high-speed data transmission, but, in the case of a communication system in a high-speed mobile environment such as a vehicle or a train, there occurs a case in which a transmitted signal goes beyond the assumption of a time-invariant channel characteristic and experiences fading in terms of time or frequency. In this case, the time variation of a channel cannot be ignored. Accordingly, in such a case, Channel State Information (CSI) acquired from packet headers (long training symbols) cannot be trusted. If a symbol period is not sufficiently long, a channel in each subcarrier band experiences frequency selective fading and, therefore, CSI acquired by interpolating pilot symbols inserted into a data symbol can not be trusted.